Are your faces enclosed? If they weren't enclosed, the automated software would try to enclose them. Pretty much what you see is what you would expect to get. As long as you follow the guidelines for the intended material.

I learned a long time ago the wisest thing I can do is be on my own side, be an advocate for myself and others like me. -Maya Angelou
michael@shapeways.com Community Advocate

Your box is large for a SW object and the walls are thick. If you go to your Model page and Edit it it will tell you the Volume. Then you can use the prices on the Materials pages to figure out the cost.